The assessment of body composition, including measurement of fat and fat-free mass, provides physicians with important information regarding physical status. Excess body fat has been associated with a variety of disease processes, such as cardiovascular disease, diabetes, hypertension, hyperlipidemia, kidney disease, and musculoskeletal disorders. Low levels of fat free mass have been found to be critically adverse to the health of certain at-risk populations, such as infants, the obese, and the elderly.
Similarly, body composition has been shown to be useful as a diagnostic measurement for the assessment of physical status. Disturbances in health and growth, regardless of origin, almost always affect body composition in newborns and infants. For example, for very low birth weight infants, body composition and variation in body composition are relevant both in determining infant energy needs and in evaluation of health progression and physical development.
A variety of methods are currently used in the assessment of body composition. One common method is a skin fold measurement, typically performed using calipers that compress the skin at certain points on the body. While non-invasive, this method suffers from poor accuracy on account of variations in fat patterning, misapplication of population specific prediction equations, improper site identification for compressing the skin, poor fold grasping, and the necessity for significant technician training to administer the test properly.
Another method employed is bioelectric impedance analysis (“BIA”). Bioelectric impedance measurements rely on the fact that the body contains intracellular and extracellular fluids that conduct electricity. In particular, BIA involves passing a high frequency electric current through the subject's body, determining the subjects'measured impedance value, and calculating body composition based on the subject's measured impedance and known impedance values for human muscle tissue. However, this method can be greatly affected by the state of hydration of the subject, and variations in temperature of both the subject and the surrounding environment. Moreover, BIA has not been successfully applied with infant subjects.
The most common method used when precision body composition measurements are required is hydrostatic weighing. This method is based upon the application of Archimedes principle, and requires weighing the subject on land, repeated weighing under water, and an estimation of air present in the lungs of the subject using gas dilution techniques. However, hydrodensitometry is time consuming, typically unpleasant for the subjects, requires both significant subject participation and considerable technician training and, due to the necessary facilities for implementation, is unsuitable for clinical practice. Further, the application of hydrodensitometry to infant, elderly, and disabled populations is precluded by the above concerns.
One technique offering particular promise in performing body mass measurement is the use of air displacement plethysmography. Air displacement plethysmography determines the volume of a subject to be measured by measuring the volume of air displaced by the subject in an enclosed chamber. Volume of air in the chamber is calculated through application of Boyle's Law and/or Poisson's Law to conditions within the chamber. More particularly, in the most prevalent method of air displacement plethysmography used for measuring human body composition (such as disclosed in U.S. Pat. No. 4,369,652, issued to Gundlach, and U.S. Pat. No. 5,105,825, issued to Dempster), volume perturbations of a fixed frequency of oscillation are induced within a measurement chamber, which perturbations lead to pressure fluctuations within the chamber. The amplitude of the pressure fluctuations is determined, and used to calculate the volume of air within the chamber using Boyle's Law (defining the relationship of pressure and volume under isothermal conditions) or Poisson's law (defining the relationship of pressure and volume under adiabatic conditions). Body volume is then calculated indirectly by subtracting the volume of air remaining inside the chamber when the subject is inside from the volume of air in the chamber when it is empty.
Once the volume of the subject is known, body composition can be calculated based on the measured subject volume, weight of the subject, and subject surface area (which, for human subjects, is a function of subject weight and subject height), using known formulas defining the relationship between density and human fat mass. For example, Siri's equation defines fat mass asPercent Fat Mass=((4.95/Density)−4.5)*100)where Density is defined assubject weight/subject volume.Similarly, Brozek's equation defines fat mass asPercent Fat mass=((4.57/Density)−4.142)*100)where Density is defined assubject weight/subject volume.
In contrast to hydrodensitometry, air displacement plethysmographic methods generally do not cause anxiety or discomfort in the subject, and due to the ease and non-invasiveness of the technique, can be applied to subjects for whom hydrodensitometry is impractical. For example, co-pending U.S. patent application Ser. No. 10/036,139, issued as U.S. Pat. No. 6,702,764, entitled Apparatus And Methods For Plethysmographic Measurement of Infant Body Composition, applied for by Philip Dempster, and filed on Dec. 31, 2001, describes apparatus and methods for plethysmographic measurement of body composition of infant subjects.
However, plethysmographic systems require very accurate measurements of volume to yield valid body composition results. In particular, plethysmographic measurement of infant body composition requires even more accurate measurement of volume given the higher metabolic activity of infant subjects as a proportion of body size, and the longer measurement periods required for infants on account of larger breathing artifacts. Due to this required accuracy of volume measurement, current plethysmographic measurement systems, while effective at measuring the volume of inanimate objects, have suffered from secondary effects that limit the accuracy of those systems with human subjects. For example, accumulation of water vapor and CO2 in the measurement chamber can significantly affect results on account of the differing adiabatic compression properties of triatomic gasses (such as CO2 and H2O) and diatomic gasses (such as O2 and N2). Similarly, variations in chamber temperature due to body heat produced by a test subject may also affect the accuracy of volume measurement.
Further, the composition of air within the measurement chamber has an effect on the comfort and safety of the test subject. Specifically, accumulation of CO2 beyond certain levels may adversely affect the infant subject. Thus, current plethysmographic systems that do not account for accumulation of triatomic gasses tend to be less suitable for determining infant body composition.
In view of the foregoing, it would be desirable to provide a plethysmographic measurement chamber that prevented the accumulation of water vapor and CO2 in the chamber, resulting in improved accuracy of body composition measurement.
It would further be desirable to provide a plethysmographic measurement chamber and air circulation system that addressed variations in chamber temperature on account of body heat produced by the test subject.
It would further be desirable to provide a plethysmographic measurement chamber and air circulation system that maintained a safe and comfortable air composition for infant test subjects.